Flood inundation in the Lancang-Mekong River Basin: Assessing the role of summer monsoon

Abstract

• Monsoons have positive impacts on inundation frequency, area and depth. • Inundation timing is either advanced or delayed when monsoons strengthen. • A change of monsoon can cause on average up to 14.2% change in inundation. • Different monsoons dominate different inundation characteristics. Although it is well known that precipitation and flood pulses in the Langcang-Mekong River Basin (LMRB) are largely impacted by monsoons, it is unclear to what extent flood inundation characteristics (i.e., inundation frequency, depth, area, and timing) in the basin respond to different monsoon types and monsoon combined effect, i.e., the Indian summer monsoon (ISM), the Western North Pacific Monsoon (WNPM), and their combined effect (ISWN). In this study, flood inundation in the LMRB during 1967–2015 was simulated by a hydrological-hydrodynamic model, from which the inundation characteristics were extracted and calculated. The monsoon impact on inundation characteristics was then quantified using the slope from linear regression model. The results show the monsoons and the ISWN overall have a positive impact on inundation frequency, depth, and area, while the inundation timing is usually advanced when the WNPM or the ISWN strengthens but delayed when the ISM strengthens. On average, a unit change in different monsoons can cause, 7.7%–14.2% change in inundation frequency, 5.3%–8.1% change in inundation depth and 4.3 days–5.8 days change in inundation timing for depth, which can also lead to 1.0%–4.3% change in inundation area and 2.8 days–3.8 days change in inundation timing for area. Also, the relative contributions of different monsoons and spatial distributions of the dominant monsoon were discussed. The results indicate different monsoons regulate different inundation characteristics, and suggest the coexistence of monsoon impacts. If the impact of the ISWN is ignored, the WNPM will play a more important role than the ISM in affecting the inundation.